To suppress noise in small electric motors, particularly fractional horsepower permanent magnet direct current motors, a varistor may be electrically connected across the commutator segments of the motor.
The properties of a varistor may be described by its E1 and E10 levels. The E1 level is the voltage at which 1 milliamp is conducted and the E10 that at which 10 milliamps are conducted. The E1 and E10 levels are temperature dependent and will drop as the varistor temperature rises. For efficient operation of the motor it is necessary to have stable E1 and E10 levels, and in particular to keep the E10 level above the normal operating voltage of the motor otherwise too much current will be shunted through the varistor. Many small PMDC motors are used in high temperature environments, typically 80 degrees centigrade, and so the temperature stability of the varistor becomes particularly important. Also, under stall conditions a high current passes through the motor and localised heating at the commutator will heat up the varistor. If the varistor is not able to dissipate heat fast enough it may overheat and eventually crack or explode.